The present invention relates generally to the manufacture of hot rolled steel strip from continuous slab casters and, more particularly, to a method and apparatus for increasing the productivity of the mill. Use of a continuous caster greatly reduces the size and expense of the plant due to the elimination of ingot casting, reheating and blooming and slabbing mills.
It is known to continuously cast steel into thin slab, on the order of 2 to 21/2 inches (.apprxeq.50-65 mm) thick, and to subsequently cut and process the lengths of cut material directly to strip on a hot finishing mill train. A buffer, in the form of a furnace, is usually provided between the casting station and the rolling mill to keep the cast material hot prior to rolling and to accommodate the difference in casting and rolling speeds. The cast stand is cut into a desired slab length downstream of the caster and the slab is either maintained in an elongated flat shape or it is coiled and held in the furnace buffer prior to finish rolling. U.S. Pat. Nos. 4,829,656 to Rohde and 4,698,897 to Frommann, et al. are exemplary of these above-mentioned known processing techniques for producing hot strip to finish gauge from a continuous caster.
It has been common practice to roll the cast slabs from the buffer furnace into finish hot strip on a multi-stand continuous finishing mill line. Single stand reversing finishing mills with coil boxes on upstream and downstream sides of the mill have also been proposed as noted in the aforementioned U.S. Pat. No. 4,698,897 to Frommann, et al. and also in U.S. Pat. No. 5,150,597 to Sekiya, et al. to further process the material into finish strip gauge. In U.S. Pat. No. 4,998,338 to Seidel, et al., thin continuously cast slab, on the order of 2-21/2 inches thick (.apprxeq.50-65 mm), is finish rolled in a 3 or 4 stand reversible finishing mill train with a coil box located at the downstream end thereof. The mill is supplied with metal from one or two continuous casters. The cast strands are cut into slab length after solidification and then heated in a soaking furnace to a selected rolling temperature prior to rolling in the reversible finishing train.
There has been a trend in recent years toward the installation of compact mills where a thin slab caster is directly connected to a continuous hot finishing mill. While these proposed mill configurations offer many advantages over conventional continuous hot strip mills, such as reduced space requirements, lower installation costs and lower man-hours per ton, there are, nevertheless, a number of significant disadvantages present in these compact mills. Among such disadvantages are low productivity (about 800,000 tons/year); and unbalanced production between the continuous caster and the finishing mill train (800,000 tons/year per caster and 3,000,000 tons/year for the finishing mill). In addition, the continuous finishing trains are expensive and require high installed horsepower to reach thin finished gauges. A common final strip thickness of 2 mm is oftentimes difficult to obtain in these mills. Finally, these prior mills are limited to a relatively narrow range of steel grades.
The present invention overcomes the disadvantages and shortcomings of the prior art by providing a compact hot strip mill and continuous slab caster which significantly increases the annual production over conventional strip slab casters.
The present invention further provides a less complex mold design in the continuous caster and increases the number of steel grades which may be cast. In addition, the invention permits the production of shorter length slabs compared to conventional strip cast slabs, thus, making it easier to shift slabs when operating a multi-strand caster along with the possibility of employing shorter length soaking/buffer furnaces.
Still further, the present invention provides a finishing mill configuration having fewer mill stands than conventional continuous strip mills with lower installed motor horsepower. The finishing mill component of the invention is supplied with a greater ton/hour output of steel from the caster to provide a more balanced production and take advantage of the heretofore greater output capabilities of the finishing mill. As a result of the higher output of the caster, the invention provides reduced bar-to-bar waiting time which allows the mill to run at steady state conditions for greater periods of time than heretofore possible.